Interferometric synthetic aperture radar (IFSAR) is a promising technology for a wide variety of military and civilian topographic modeling requirements. IFSAR extends traditional two dimensional SAR processing to three dimensions by utilizing the phase difference between two SAR images taken from different elevation positions to determine an angle of arrival for each pixel in the scene. This angle, together with the two-dimensional location information in the traditional SAR image, can be transformed into geographic coordinates if the position and motion parameters of the antennas are known accurately.
Radar systems have been used for decades because of their ability to penetrate fog, clouds, and darkness. A synthetic aperture radar (SAR) typically moves a single antenna along a path and collects a single channel data from many positions along the path. Because scanning is provided by the movement of the antenna, a small SAR antenna has the ability to have the resolution of a very large antenna. SAR systems currently have the ability to make images having resolution on the order of less than a meter.
In contrast, an IFSAR typically comprises a pair of antennas, which moves along a path and collects at least two channels of data from many positions along the path or alternatively has a single antenna but moves along two different paths. IFSAR has the ability to make elevation models from extremely long stand-off ranges. For example, a system in low-Earth orbit may provide elevation accuracy on the order of a meter. However, because a SAR uses range to measure ground distance, it cannot be operated looking directly down on a target, so shadowing (where a tall object hides whatever is behind it) and layover (where the reflection from the ground in front of a tall object arrives at the antenna at the same time as a reflection from the object) are significant problems. Unfortunately, when multiple reflectors occur within a single SAR range bin, incorrect elevations are obtained as a result of producing a weighted average from the multiple reflectors. Moreover, a "front porch" effect can be obtained that not only mischaracerizes elevation, but also distorts the horizontal extent of the topographic feature of interest.
An overview of IFSAR is provided by D. Bickel et al., "Design, Theory, and Applications of Interferometric Synthetic Aperture Radar for Topographic Mapping," Sandia National Laboratories Report SAND96-1092, May 1996, available through DOE/OSTI, which is incorporated into this patent specification by reference thereto.
Presently, optical stereo height measurements are used almost exclusively to measure building heights. Such a process uses stereo optical points to determine height. The difficulty with this process is that it requires tedious selection of points by human operators. On the other hand, the present IFSAR technique uses the phase information in the returning signals and can be automated or nearly automated with a significant reduction in processing time.